Disk drives are well known in the computer art for providing secondary mass storage with random access. A disk drive essentially comprises one or more magnetic data storage disks rotating on a spindle by a spindle motor, within an enclosed housing. A magnetic transducer head is placed on an actuator arm system and positioned very closely to each data storage surface by a slider suspended upon an air bearing. Servo information is typically written in servo sectors which interrupt data sectors or blocks.
Servo information provides a servo control loop in the disk drive with head position information to enable a head positioner mechanism, such as a rotary voice coil motor, to move the actuator, and therefore the head, from track to track during random access track seeking operations, and to maintain the head in proper alignment with a track centerline during track following operations when user data is written to or read from the available data block storage areas of the disk surface. As such, the servo control loop is used to control head positioning as the head is being moved transversely across tracks by the actuator, and to cause the head to remain over a particular data track as the disk spins. The servo loop controls the acceleration of the head, which results from a force supplied by the electric motor on the actuator.
The inputs to the servo system are readings of head position made by the head itself. The head position is determined from servo information written directly onto the disk e.g. by self-servo writing (SSW) or by a servo track writer (STW), as part of the manufacturing process. The servo information may include the track number as well as an indication of how far the recording head is from the track center line. That is, a certain amount of information on each track is reserved for indicating position. As the head passes over the indicators, the track over which the head is sitting is determined by the head itself and supplied to the servo system. The indicators are typically at regularly spaced locations.
In an ideal disk drive system, the tracks of the data disk are non-perturbed circles situated about the center of the disk. As such, each of these ideal tracks includes a track centerline that is located at a known constant radius from the disk center. In an actual system, however, it is difficult to write regularly spaced and non-perturbed circular tracks to the data disk. That is, problems, such as vibration, bearing defects and/or inaccuracies in the servo writing process (among other things) cause repeatable and non-repeatable runout while writing track servo sectors, leading to mis-positioned and/or perturbed tracks. The repeatable and non-repeatable runout become written in runout (WRO) when the servo sectors are written.
In one case, the WRO represents mis-positioning of the servo sectors that results in tracks that are written differently from the ideal radially regularly spaced tracks. That is, at least some of the tracks are spaced too close to, or too far apart from, one another. This mis-positioning is referred to as “squeeze” which limits the Off Track Read Capability (OTRC) of the disk drive and can cause encroachment (overwrite), leading to data loss.
Further, magneto-resistive (MR) heads in modern disk drives have a position difference between a reader element and a writer element therein that necessitates use of microjog techniques as is well known to those skilled in the art. The amount of microjog required varies from head to head, and also varies across the stroke due to track skew angle effects. It is not uncommon to have a microjog that is as great as 13 tracks. In such a case, when data is written to a track with the head writer, then the head must be moved 13 tracks to one side for reading data back from that track with the head reader. Typically, a microjog calibration routine determines the microjog at several locations across the head stroke from disk inner diameter (ID) to outer diameter (OD), and then calculates a microjog profile. The microjog profile is then used to move the head, after writing data to a track with the head writer, to read the data back from that track with the head reader. A separate microjog profile is required for each head.
The microjog calibration routine assumes that track density (e.g., tracks per inch or TPI) is kept constant across the head stroke. Small changes in track density due to squeeze have large effects on microjog. For example, where there is 7-tracks of microjog, a 5% change in track density due to squeeze in an area of a data disk, translates into a 35% off-track microjog error condition when attempting to read back data from a track. The amount of track density change is accumulated over the 7 tracks (5*7=35) to cause the 35% microjog error. This problem is becoming increasingly significant as track densities are increased. A side effect of increased track densities is that servo track spacing errors and resulting track squeeze are more significant to data integrity.
There is, therefore, a need for a method to compensate for microjog errors induced by localized track squeeze in disk drives in order to prevent data storage integrity problems.